1. Field of the Invention
The present invention relates to a fluorescence tomography apparatus which applies excitation light to a region of an object to be observed, detects fluorescence emitted from the object, and acquires a fluorescence tomographic image of the region.
2. Description of the Related Art
In recent years, optical tomography apparatuses which acquire tomographic images by using light have been used in the field of medical diagnosis. The optical tomography apparatuses using confocal optical systems and the optical tomography apparatuses using time-resolved measurement systems are already in actual use.
In addition, commercialization of the optical coherence tomography (OCT) apparatuses has been recently proceeding. In the OCT apparatuses, low-coherence light is emitted from a light source realized by a super luminescent diode (SLD) or the like, and split into measurement light (light applied to an object for measurement) and reference light. The frequency of the reference light or the measurement light is slightly shifted by using a piezoelectric element or the like, and the measurement light is applied to a region to be observed, so that light reflected from a position at a desired depth in the region to be observed interferes with the reference light, and information on tomography is obtained by measuring the intensity of the interference light by heterodyne interferometry. It is possible to obtain information at a depth at which the optical path length of the reference light coincides with the optical path length of the measurement light, by slightly moving a movable mirror arranged in the optical path of the reference light so as to slightly change the optical path length of the reference light.
Further, development of an ultrasound-modulated optical tomography apparatus has also been proceeding recently as disclosed in “Ultrasound-modulated Optical Computed Tomography of Biological Tissues”, by Jun Li and L. V. Wang, Applied Physics Letters, Vol. 84 (2004), No. 9, pp. 1597-1599. In such an ultrasound-modulated optical tomography apparatus, an ultrasonic wave and light are concurrently applied to a region to be observed, and an ultrasound-modulated-light tomographic image is obtained on the basis of ultrasound-modulated reflected light, which is generated by reflection from the region influenced by the ultrasonic wave.
Furthermore, fluorescence tomography apparatuses using a confocal optical system are being actually used. In such fluorescence tomography apparatuses, excitation light is applied to a region to be observed, and a fluorescence tomography image is obtained by detecting fluorescence emitted from the region. Moreover, fluorescence tomography apparatuses using a time-resolved measurement system are disclosed in U.S. Pat. No. 6,174,291.
The optical tomography apparatuses using a confocal optical system can obtain high-resolution optical tomographic images. However, it is impossible to acquire a fluorescence tomographic image of a region which is to be observed and extends to a depth of 0.5 mm or greater from a surface of the region.
On the other hand, the fluorescence tomography apparatuses using a time-resolved measurement system can obtain fluorescence tomographic images of regions at depths from zero to several millimeters from a surface of the region. However, in this case, the resolution is at most approximately 1 mm, and insufficient.
In particular, in order to diagnose the extension of a lesion in the field of medicine, there are strong demands for a fluorescence tomography apparatus which can obtain a high-resolution fluorescence tomographic image of a region extending to a depth of several millimeters.